Telomere shortening causes liver cirrhosis in a subset of patients with telomerase mutations and predisposes to the development of liver cirrhosis and liver failure in acquired conditions. A major gap in the telomere field is our poor understanding of the mechanisms that are activated by short telomeres and drive the disease process. Consequently, no therapeutic interventions exist currently to prevent telomere-related liver disease due to an insufficient understanding how telomere shortening compromises cellular health. We have recently discovered that telomere shortening in telomerase knockout mice (TKO) impacts cellular metabolism in part through a p53-dependent down-regulation of PGC1a/, co-activators that regulate mitochondrial biogenesis/function. This leads to a decrease in mitochondrial numbers and function. However, whether telomere-mediated p53 activation leads to metabolic changes during normal aging, the precise pathways by which p53 impacts metabolism, and the relevance of these metabolic changes for the pathogenesis of telomere-related diseases such as liver cirrhosis remains to be defined. In this grant proposal, we address these critical questions experimentally through a series in experiments in the liver. The proposal is based on our recent observation that TKO and old wild-type mice liver tissues have reduced levels of all seven Sirtuins, a class of enzymes highly implicated in diseases. How these enzymes are regulated is not known, however, their down-regulation is well understood to lead to many disorders. In TKO mice, the down-regulation of Sirtuins is partially p53-dependent. Furthermore, our preliminary studies indicate that p53 regulates the mitochondrial Sirtuins (Sirtuin 3, 4, 5) in a PGC1a-dependent manner at the transcriptional level, and the non- mitochondrial Sirtuins (Sirtuin 1, 2, 6, 7) through miRNAs at the translational level. Our hypothesis is that telomere shortening-induced p53 down-regulates Sirtuins and that this Sirtuin repression contributes to telomere-dependent liver disease. We will test this hypothesis through three approaches: First, to determine the causal relationship between p53 and Sirtuin expression during aging, we will delete p53 in liver tissue of old mice, use a mouse model of premature aging due to hyper-active p53 (p53+/m mutant mice) and then assay changes in Sirtuin levels and function (Aim 1). Second, we will focus on how altered p53 activity impacts the translationally regulated Sirtuins. We have identified four p53-dependent miRNAs in liver tissue of TKO mice that bind to Sirtuins. We will use gain/loss-of-function studies to test whether these candidate miRNAs repress individual non-mitochondrial Sirtuins and determine the consequence of their deletion for liver function (Aim 2). Finally, to examine the pathogenic impact of Sirtuin down-regulation on liver cirrhosis, we will overexpress Sirt1 and determine whether restoration of Sirt1 is sufficient to improve liver cirrhosis induced by short telomeres. Together, these experiments will provide new insights in how telomere shortening impacts metabolism and predisposes to liver disease.